1. Field of the Invention
The present invention relates to a semiconductor device.
2. Background Art
Recently, with miniaturization and complication of a semiconductor device, a multilayer interconnection structure is adopted. In order to form the multilayer interconnection structure, it is necessary to repeatedly perform plasma CVD (Chemical Vapor Deposition), RIE (Reactive Ion Etching) or the like on a MOS transistor. Since these treatments are for performing deposition or etching utilizing electrical polarities, a gate electrode and a substrate are charged. Thereby, there is a possibility that an excessive electric field is applied to a gate insulating film and the gate insulating film is destroyed. Further, after the semiconductor device is completed, there is a possibility that the gate insulating film is broken down by human electrostatic discharge (ESD), for example.
A protective diode is formed for protecting a MOS transistor from such plasma damage or electrostatic damage. The protective diode is connected between the gate electrode and the substrate so as to be reversely biased when a gate voltage is applied thereto. Thereby, the protective diode prevents the gate electrode from being applied with a voltage more than the reverse break-down voltage.
By the way, recently, a technology for forming a MOS transistor or memory device on SOI (Silicon On Insulator) is developed. Since the embedded oxide film (also referred to as “BOX” (Buried Oxide) layer) is getting very thinner (for example, 10 nm to 300 nm), not only the protection of the gate oxide film, but also the BOX layer can be broken down by the plasma damage or the electrostatic damage (see Publication of Japanese Patent Application No. 2002-538598 and International Publication No. W00/44049 pamphlet).
Accordingly, a semiconductor device in which, in the case where a MOS transistor or memory device is formed on an SOI substrate, not only the gate oxide film, but also the BOX layer can be protected from plasma damage or electrostatic damage is desired.